Inlet port design to improve scavenging in overhead valve two-stroke engine

ABSTRACT

(In a Two Stroke engine, there is very little opportunity for scavenging residual exhaust gas, without using impractical exhaust pipe tuning.) Our design utilises momentum air-filling of the cylinder. We use overhead poppet valves, but we have positioned the inlet valve at an angle, and partly recessed it. The angled upstream side of the valve head and the wall of the recess direct airflow. The eminence positioned in the inlet port augments this by a Coanda effect. Air is thus directed alongside and under the exhaust gas, to promote scavenging and optimise volumetric efficiency. This also creates a combustion chamber in the coolest part of the cylinder head, an important consideration. Complex surfaces, liable to overheat, have been avoided. The design is simple and robust.

BACKGROUND

The usual internal combustion engine is of the so-called Four Stroke type. In this engine, one whole revolution is concerned with pumping out exhaust gas and drawing in combustible air. The volume of clean air sweeping through the engine approximates the piston displacement volume, ie Volumetric Efficiency (VE) ˜100%.

By contrast the Two Stroke engine, in a single revolution, burns the air-fuel mix, empties the cylinder of exhaust gas, and admits the air and fuel charge. Air has to be driven in by some form of compressor. Most of the exhaust gas exits under its own pressure, but final residual gas has to be cleared by incoming air. Only about one third of a revolution is available for the incoming air to sweep out the exhaust gases. Sophisticated exhaust tuning may be used to increase exhaust exit and air delivery, commonly restricted to special application (eg competition motor cycle engines). It is usually less efficient than the Four Stroke. However, because it fires twice as often as the Four Stroke, it may be more powerful for its cylinder capacity and is likely to have a better power to weight ratio.

In the Two Stroke, the usual way to clear (scavenge) the residual exhaust gas is to drive air in through ports in the cylinder lining, expelling exhaust gas either via similar ports, or via overhead valves. This has at least two disadvantages

-   -   1) Compressed air in a chamber has to wait (without net         direction) for the port to open and then accelerate across the         port. Gas flow in this setting must be suboptimal. VE is         impaired.     -   2) Since the piston is also acting as a sleeve valve, it has to         be rather long, adding to the height, and therefore also weight,         of the whole engine.     -   3) Ports may contribute to piston ring wear.

Scavenging is of critical importance because it involves removal of spent gas and replacement with combustible air. It is our contention (1) that inlet tracts like those in a modern Four Stroke, with overhead valves, would permit momentum (or ‘ram’) filling of the cylinder and thereby improve scavenging.

The problem is that in a Two Stroke engine, where inlet and exhaust valves are open simultaneously, the air entering via an overhead valve short-circuits the scavenging process and passes straight into the exhaust. We observe that this stream of air does not even draw in exhaust gas, but rather acts as a curtain to exclude it, holding it in the cylinder.

Another aspect is combustion itself. Much attention is given to rapid mixing of the air and fuel, both before and during combustion, requiring the incoming air to have turbulence. Moreover, air is sometimes introduced through some form of nozzle to form a jet. But a jet accelerates the air stream, resulting a pressure drop. This draws the spent gas into the clean air stream, impairing VE.

We think that the scavenging process should be separated from the turbulence required with combustion. We believe (2) that scavenging is improved if, upon entry, the air flows as smoothly as possible and expels the exhaust without mixing. Later in the cycle, the air becomes highly turbulent as the piston crown approaches the cylinder head. Fine control of fuel injection is then essential for optimal combustion.

A final point: the concentration of heat in the small combustion chamber of a diesel engine causes differential expansion (heat stress) in that region. This is worse in a Two Stroke engine, and further aggravated in a turbo-charged unit. Positioning of the combustion chamber, and its cooling, must be carefully considered.

We therefore sought an inlet tract design that would (1) deliver a maximal air mass, and (2) direct this air smoothly around and under the residual exhaust gas so as to expel it with a minimum of mixing. Our design necessitated a recess in the cylinder head, at the relatively cool inlet ports, which fortuitously was in a satisfactory position and shape to function as the combustion chamber.

SUMMARY

At, say, 2500 revs per minute there are 40 milliseconds per revolution. In a Two Stroke engine, the cycle of valves starting to open, being effectively open, then finish closing, occurs over about 15 millisec. But they are effectively wide-open perhaps only 10 millisec. In order to optimise air entry over this short interval, air should be moving as a rapidly moving column. (We advocate momentum filling to mimic modern Four Stroke engine practice). For optimal scavenging, the incoming air must slip alongside and under the residual exhaust gas and expel it. Mixing is undesirable.

We start with a flat cylinder head face. We have designed an inlet tract which approaches at 90° and curves through approximately 28° toward the exhaust valve. The valve seat is set at the same angle to the cylinder head face (say, use 28° throughout now, though a slightly smaller angle can be used). The seat is thus recessed in such a way that when the valve opens, approximately one third of the opening is shielded within the recess. The air is then directed off the upper surface of valve head, away from the exhaust valve. To augment this direction of movement, a minor elevation is cast in the port immediately above the valve seat, on the side opposite the exhaust port. This smoothly curved eminence is gently angled and draws air away from the exhaust side (applying a Coanda effect). Thus, airflow is directed down the cylinder sides opposite the exhaust port, onto the piston crown, pushing exhaust gas toward the exhaust port. A depression in the piston crown matches the recess beneath the inlet valves, forming part of the combustion chamber. A ramp on its exhaust side, directs central air upward.

This design

-   -   1) Gets a large mass of air into the cylinder because of the         high entry speed.     -   2) Directs air away from the exhaust port     -   3) Directs air under the exhaust gas, minimising mixing     -   4) Promotes scavenging     -   5) Positions the combustion chamber in the coolest part of the         cylinder head.

All of the above considerations correct weaknesses in usual Two Stroke engine design.

DRAWING

1. The drawing shows a section through the cylinder head, with piston crown some distance from Bottom Dead Center. Both valves are part open.

Air is forced in, onto the back of the poppet valve head but can only escape into the cylinder in region F. It cannot escape in region A because of the position of the valve in the recess. Thus, there is no short-circuit of air into exhaust port at E. The Coanda curve, B, above the valve seat, drags airflow towards region F. Air streams down at F, crosses the piston top, and moves up towards the exhaust valve. Where it dips into the piston part of the combustion chamber, it is directed to the centre of the cylinder.

1.1. The cylinder head is shown in plan view, from the underside. The small circle near-centre is the injection nozzle.

1.2. The three-dimensional drawing simply gives an idea of how the inlet valves must enforce the direction of airflow. The recess whose wall stops airflow into the exhaust port can be seen.

DESCRIPTION (DETAIL)

The prototype cylinder head was machined from aluminium alloy using a CNC machine.

In the experimental engine, we took a Lister-Petter AC1 diesel engine. We replaced the cylinder head with the experimental one.

The induction tracts were downdraught type with a final curve of 28° in the prototype, so that the apparent air entry was towards the exhaust valve. However, because the valve seat was recessed at 28°, when fully open, the valve head directed airflow away from the exhaust. Some 65% of the valve head circumference directed air to spill in this way into the inlet side of the cylinder. The remaining 35% of air is redirected from its natural path by the wall of the recess, to spill sideways (away from the open exhaust ports). We found that a small eminence tapering to the valve seat on the valve-stem side exerted a Coanda effect, pulling airflow to that same side. Thus, air streamed down into the inlet side of the cylinder forcing exhaust gas up toward the exhaust valve. Turbulence, intake air jets, and mixing were considered undesirable and the design aimed to avoid these.

There were two inlet and two exhaust valves, with overhead cams driven from a timing wheel mounted on the original hand-crank position. The valve timing was slightly asymmetric.

The exhaust outlets merged in a 28 mm diameter turbo-charger (Renault Clio) and the compressor fed cooled air (approx 10° C.) into the inlet tracts via a plenum chamber of 500 mL capacity.

Computational fluid dynamic considerations suggested that by positioning the combustion chamber in the piston crown below the recess in the head, and shaping it into a scoop, air would be swept into the central part of the cylinder, expelling exhaust gas.

Considerable effort and resources went into retuning the Delphi high pressure fuel injection system (also taken from a Renault Clio) to optimise performance and minimise emissions.

Historically, Two Stroke engines have made use of sophisticated and untransferable exhaust tuning. In these systems, the energy of the exhaust gas is used (in the final analysis) to suck air into the exhaust system and then push this extra air back into the cylinder, at the moment of valve closure. An alternative system is to use a compressor to drive air into the cylinder via inlet ports in the cylinder liner wall, and out through overhead valves. This seems to be a good arrangement and variations have been introduced to ensure scavenging of centrally lying gas. However, all rely on jets of air, ignoring the fact that the accelerated air is of high-velocity, low-pressure which mixes with exhaust gas. Further, though the air is pressurised, it is not directed or exhibiting net movement until the moment the inlet port opens. This system is therefore of limited use as engine speed increases, with only milliseconds to perform scavenging. Additionally, ports in the cylinder wall increase piston ring wear and hence engine servicing frequency.

Air could be better introduced by momentum filling down an inlet tract as used in high performance Four Stroke engines.

As turbo-chargers have been refined, it has been possible to use exhaust energy to feed the compressor, but overhead inlet valves do not seem to have been used. (We believe a Ricardo design may have used a poppet inlet valve fitted with a shield to direct airflow, but this is a clumsy idea with none of the mutually beneficial features of our concept. We could not confirm its existence.) Certainly, the idea of minimising turbulence to promote scavenging is to our knowledge novel. We think we have achieved this by masking part of the inlet port in an angled recess. We find that air is automatically directed alongside and under the exhaust gas, promoting better scavenging with less mixing. Thus, the material available at the moment of combustion may be assumed to be more pure air. This design optimises VE. An incidental, but important, benefit is that the recess becomes the combustion chamber. This is therefore sited in the coolest part of the cylinder. 

1) the angled position of the poppet valve in its recess directs air away from the exhaust port, improving scavenging; 2) the wall of the recess, adjacent to the exhaust valve, prevents air from short-circuiting into the exhaust; 3) the small eminence above the valve seat on the valve stem side exerts a Coanda effect and drags air to that side of the port; 4) airflow is of low turbulence, reducing mixing of air with exhaust gas; 5) the effectively continuous high speed column of air in the inlet tract contributes to greater volumetric efficiency of the engine; 6) the position of the combustion chamber in the coolest part of the cylinder head reduces heat stress in the machined parts; 7) the design is simple and robust. 